The present invention relates to cordierite bodies, and especially the kind with properties suitable for use in exhaust after-treatment applications, particularly diesel exhaust filtration.
Cordierite ceramic bodies, especially such formed as honeycomb multicellular structures, find employment in a number of high temperature applications such as catalytic converters, NOx adsorbers, electrically heated catalysts, molten metal filters, regenerator cores, chemical process substrates, catalysts for hydrodesulfurization, hydrocracking, or hydrotreating, and filters such as diesel particular filters.
In diesel exhaust filtration, cordierite, being a low-cost material, in combination with offering a low coefficient of thermal expansion (CTE), has been the material of choice. Porous cordierite ceramic filters of the wall-flow type have been utilized for the removal of particles in the exhaust stream from some diesel engines since the early 1980s. A diesel particulate filter (DPF) ideally combines low CTE (for thermal shock resistance), low pressure drop (for engine efficiency), high filtration efficiency (for removal of most particles from the exhaust stream), high strength (to survive handling, canning, and vibration in use), and low cost. However, the combination of high filtration efficiency, high strength, and very low pressure drop has proven elusive with cordierite DPFs.
Typical commercially available cordierite DPFs have about 48% porosity, a median pore diameter of about 12 μm, and a broad pore size distribution. While such DPFs have adequate filtration efficiency and strength, their pressure drop when loaded with carbon soot is high. This occurs as a rapid increase in pressure drop during the loading of the first gram/liter of carbon soot. The initial soot is believed to enter into the pores of the ceramic and reduce the permeability through the filter walls. Subsequent to the first gram/liter of soot, the increase in pressure drop with soot loading becomes more gradual.
Reduction in soot-loaded pressure drop has recently been reported for higher-porosity (59%) filters with a larger median pore size (25 μm) (SAE Technical Paper 2002-01-0322, “SiC and Cordierite Diesel Particulate Filters Designed for Low Pressure Drop and Catalyzed, Uncatalyzed Systems,” Hashimoto et al.). However, strengths for filters with such high porosity and coarse pore size are so low that especially careful handling is required. Furthermore, these filters still exhibit an undesirably high rate of increase in pressure drop with the addition of the first gram/liter of soot, although not as severe as for previously available lower-porosity filters with finer pore size. Work by the present investigator has shown that further reduction in pressure drop is possible when median pore size is increased beyond 25 μm; however, such cordierite ceramic bodies have a low filtration efficiency.
It would be considered an advancement in the art to obtain cordierite ceramic bodies that combine high filtration efficiency, along with low pressure drop and improved strength. The present invention provides such bodies and method of making them.